linux-kernel - Re: spinlock contention of files->file

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Message-ID: <20131001032715.GW13318@ZenIV.linux.org.uk>
Date:	Tue, 1 Oct 2013 04:27:15 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: spinlock contention of files->file_lock

On Mon, Sep 30, 2013 at 07:02:23PM -0700, Linus Torvalds wrote:

> Shouldn't a cmpxchg() in just the dup2 code solve that?
> 
> If the old value was NULL, you'd have to repeat and go back and see if
> the open_fds[] bit had been cleared in the meantime (ie it's NULL not
> because somebody else is busy installing it, but because somebody just
> uninstalled it).

Yechh...  Under ->file_lock (in do_dup2()), hopefully?  Or you'll get
all kinds of fun with close() thrown into the game, as well...

> But yeah, I do agree that that sounds nasty and a complication I
> hadn't even thought about. dup2() does violate our normal "let's
> pre-allocate the fd slot" rule. Ugh.

Hell knows...  Descriptor handling *is* pretty well isolated these
days, so it just might be doable without disrupting the living hell
out of anything else.  fs/file.c is pretty much it - everything else
goes through it.

I've enough on my plate at the moment with fs/namespace.c and fs/namei.c,
though, and praying hard fs/inode.c doesn't enter the game.  I _know_
that fs/notify will and I'm not enjoying that for a second.  BTW, has
eparis resurfaced with any fixes for *notify/umount races?  I don't seem
to have anything related in the mailbox, but...
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